This invention generally relates to seals and more particularly relates to an extrusion-resistant seal assembly for sealing a gap defined between a first structure spaced-apart from a second structure, which first structure and second structure may be a first flange and a second flange belonging to an instrumentation column of the kind typically found penetrating nuclear reactor pressure vessels.
Before discussing the current state of the art, it is instructive first to briefly describe the structure and operation of a typical nuclear reactor pressure vessel and its associated instrumentation columns. In this regard, a nuclear reactor pressure vessel is a device for producing heat by controlled fission of fissile material contained in a plurality fuel rods grouped to form a plurality of fuel assemblies disposed in the pressure vessel. The plurality of fuel assemblies define a nuclear reactor core in the pressure vessel. The pressure vessel itself includes a shell having an open top end and a closure head sealingly capping the open top end of the shell, so that the pressure vessel may be suitably pressurized thereby. A plurality of absorber rods slidably extend into each fuel assembly for controlling the fission process therein. Liquid moderator coolant (i.e., demineralized borated water), which may be pressurized to a pressure of approximately 2,500 psia during normal operation or approximately 3,000 psia during off-normal operation (e.g., during "overpressure" transients), is caused to flow over the fuel rods disposed in the pressure vessel for assisting in the fission process and for removing the heat produced by the fission process. During operation of the nuclear reactor, heat due to fission of the fissile fuel material is carried from the fuel rods by the liquid moderator coolant flowing over the fuel rods, which liquid moderator coolant becomes radioactive as it flows over the fuel rods. The heat carried away by the liquid moderator coolant is ultimately transferred to a turbine-generator set for generating electricity in a manner well known in the art.
Penetrating the closure head are a plurality of elongated instrumentation columns, each instrumentation column having a longitudinal bore therethrough for receiving instrumentation wiring connected to an instrumentation probe located in the reactor core. The bore of the instrumentation column is typically in fluid communication with the pressurized reactor coolant circulating through the pressure vessel. Each probe is designed to measure predetermined core physics quantities (e.g., temperature, neutron flux, etc.) in the reactor core. Moreover, the instrumentation column is segmented for ease of assembly and servicing. The ends of at least some of the segments include opposing generally circular flanges, which are capable of being moved into close proximity and connected together for completing the assembly of the segmented instrumentation column. At least one of these pair of opposing flanges is located externally to the pressure vessel.
As stated hereinabove, the pressurized radioactive coolant is in fluid communication with the bore of the instrumentation column. Therefore, for safety reasons, it is prudent to provide suitable seals at the interface of the flanges to prevent leakage of the pressurized radioactive coolant from between the flanges.
In this regard, it is known that graphite is useable as a seal material because when clamped between opposing structures, it has a relatively higher compressibility than, for example, an all-steel seal. More specifically, it is known that an all-graphite seal can compress approximately 30% more than an all-steel seal. However, it is also known that graphite seals are most suitable to seal against relatively low fluid pressures of about 600 psia. When exposed to higher fluid pressures, such all-graphite seals tend to experience what is termed in the art as "blow-out". That is, when an all-graphite seal "blows-out", it tends to extrude, thereby compromising the ability of the all-graphite seal to maintain its sealing function. Therefore, although all-graphite seals can withstand relatively high compression, they tend to extrude at the relatively high fluid pressures (e.g. 2,500-3,000 psia) achievable in nuclear reactor pressure vessels. Hence, a problem in the art is to provide a suitable graphite seal for use in nuclear reactor pressure vessel instrumentation columns, the seal being capable of withstanding relatively high pressure without extrusion or "blow-out".
A graphite seal is disclosed in U.S. Pat. No. 3,564,400 titled "Nuclear Magnetic Resonance Flowmeter Employing Ceramic Tube" issued Feb. 16, 1971 in the name of Ronald L. Pike et al. The Pike et al. patent relates to a nuclear magnetic resonance (or NMR) flowmeter, and particularly to such a flowmeter in which a ceramic tube is employed as the central conduit for conveying paramagnetic fluid. This patent discloses a flange secured to an end plate by means of bolts. A graphite sealing ring is disposed between the end plate and the flange in an annular recess provided in the exterior surface of the end plate. The interior side of the flange is provided with a boss, which has dimensions comparable to those of the annular recess. When the bolts are tightened, the graphite ring is compressed in the recess between the end plate and the boss, and is distorted under pressure to flow outwardly from the recess and fill up any space between the tube and the surfaces of the end plate and the flange. Therefore, this patent does not appear to disclose a graphite seal assembly configured to resist extrusion.
Although the above recited prior art discloses a graphite seal, the above recited prior art does not appear to disclose a high pressure extrusion-resistant seal assembly for sealing a gap defined between a first structure spaced-apart from a second structure, which first structure and second structure may be a first flange and a second flange, respectively, belonging to an instrumentation column of the kind typically found penetrating nuclear reactor pressure vessels.
Therefore, what is needed is an extrusion-resistant seal assembly for sealing a gap defined between a first structure spaced-apart from a second structure, which first structure and second structure may be a first flange and a second flange, respectively, belonging to an instrumentation column of the kind typically found penetrating nuclear reactor pressure vessels.